The present invention relates generally to sewing machines of a type which sews an elongated string-shaped material, such as a tape or cord, onto a fabric (sewing workpiece) through lock stitching. More particularly, the present invention relates to an improved sewing machine in which a bobbin having an elongated string-shaped material wound thereon is disposed above a needle bar and has an increased size and which, in sewing the string-shaped material onto a sewing workpiece, allows the string-shaped material to be paid out smoothly from the bobbin by positively rotating the bobbin.
Heretofore, there have been known sewing machines of a type which includes a vertically driven needle bar, a sewing needle fixed to the lower end of the needle bar, a rotary member mounted concentrically with the needle bar and freely rotatable about the axis of the needle bar, and a guide fixed to the rotary member for guiding an elongated string-shaped material (i.e., string-shaped embroidering material, such as a tape or cord) to a sewing position of the sewing needle. The sewing machines of this type operate to sew the string-shaped material to a fabric, through lock stitching, while controlling the rotation of the rotary member in accordance with a moving direction of the fabric based on embroidery data and appropriately adjusting the orientation of the guide to optimize the direction in which the string-shaped material is guided to the sewing position of the sewing needle. One example of such sewing machines is known from Japanese Patent Application Laid-open Publication No. 2005-144056. The No. 2005-144056 publication discloses a sewing machine where, in order to permit an increased size of a bobbin (material holder) having a string-shaped material wound thereon, not only the bobbin is provided above the needle bar but also a drive means is provided for rotating the bobbin, on the basis of a tensile force acting on the string-shaped material, so that the string-shaped material can be supplied smoothly even where the bobbin has an increased size.
The sewing machine disclosed in the aforementioned No. 2005-144056 publication is briefed here. The bobbin, having the string-shaped material wound thereon, is mounted on a bobbin shaft that is supported at its opposite ends by a pair of opposed support members fixed to a machine frame, and the bobbin is supported at its opposite ends by a pair of holding members provided on the bobbin shaft. Pulley, which is driven to rotate by a drive motor provided on the support member, is provided in engagement with the lower end of one of opposed flanges of the bobbin. Thus, as the pulley is rotated by the drive motor, the bobbin is rotated. Beneath the bobbin, there are provided a first guide roller for winding therearound the string-shaped material paid out downward from the bobbin and inverting upward the wound string-shaped material, and a second guide roller for winding therearound the string-shaped material inverted by the first guide roller and re-inverting downward the string-shaped material. The first guide roller is rotatably mounted on a support arm fixed to the support member, while the second guide roller is rotatably mounted on a pivot arm pivotably fixed to the support member. Coil spring for normally urging the pivot arm upward is provided between the pivot arm and the support member. On a proximal end portion of the pivot arm, there are provided an actuating piece pivotable by the pivoting movement of the pivot arm, and a limit switch operable by the pivoting movement of the actuating piece. Driving by the drive motor is controlled in response to turning-on/off of the limit switch responsive to the pivoting movement of the actuating piece.
In sewing the string-shaped material, the string-shaped material, paid out from the bobbin, is sequentially wound around the first and second guide rollers and then directed to a machine head. As the string-shaped material is sewn onto a fabric in accordance with a progression of the sewing operation, the second guide roller is pulled by the string-shaped material to cause the pivot arm to pivot downward, so that the actuating piece pivots to turn on the limit switch. Once the limit switch is turned on in this way, the drive motor is activated, so that the bobbin is rotated to pay out the string-shaped material. When the tension of the string-shaped material has decreased due to the paying-out from the bobbin, the pivot arm is caused to pivot upward, upon which the actuating piece pivots in a direction opposite from the direction in which it pivoted at the time of the turning-on of the limit switch, so that the limit switch is turned off. Once the limit switch is turned off, the drive motor is deactivated, so that the rotation of the bobbin is stopped to stop the paying-out of the string-shaped material. Then, once the pivot arm is again caused to pivot downward as the sewing of the string-shaped material progresses, the bobbin is again rotated to pay out the string-shaped material. When the pivot arm has pivoted upward by the string-shaped material having been paid out from the bobbin by a sufficient amount, the rotation of the bobbin is stopped to stop the paying-out of the string-shaped material. Namely, during the sewing of the string-shaped material, the rotation and stoppage of the rotation of the bobbin is repeated, in accordance with a tensile force acting on the string-shaped material, to allow the string-shaped material to be paid out appropriately from the bobbin as the sewing operation progresses.
In the conventionally-known sewing machines like the one disclosed in the aforementioned No. 2005-144056 publication, as the string-shaped material wound on the bobbin is sewn onto the fabric, the pivot arm is caused to pivot downward by the second arm being pulled by the string-shaped material, so that the bobbin is driven to rotate to pay out the string-shaped material. Namely, in the sewing machines constructed to rotate the bobbin on the basis of a tensile force acting on the string-shaped material, there is provided a displacement member displaceable in accordance with the tension of the string-shaped material. Therefore, an excessive tensile force, commensurate with a load with which to displace the displacement member, would be applied to the string-shaped material. On the other hand, when the displacement member returns to the original position after the string-shaped material is paid out through rotation of the bobbin, the load is reduced. Therefore, in the sewing machines where the bobbin is rotated on the basis of the tension of the string-shaped material, an unnecessary, excessive tensile force would be applied to the string-shaped material when the string-shaped material is sewn onto the fabric, and, besides, such a tensile force would always vary. However, if the tensile force acting on the string-shaped material is not constant during the sewing of the string-shaped material, the sewing of the string-shaped material can not be performed accurately in an aesthetically pleasing manner. Thus, it has been extremely difficult for the conventionally-known sewing machines to manufacture products of uniform high quality which have a string-shaped material sewn in an aesthetically pleasing manner.